The present invention relates to a block copolymer based on conjugated diolefins and polar monomers, and to a process for the preparation of the block copolymer in the presence of catalysts of the rare earths.
The polymerization of conjugated diolefins has been known for a long time and is described, for example, by W. Hoffman, Rubber Technology Handbook, Hanser Publishers (Carl Hanser Verlag) Munich, Vienna, N.Y., 1989. Thus, for example, polybutadiene is now predominantly prepared by solution polymerization with the aid of coordination catalysts of the Ziegler-Natta type, for example based on compounds of titanium, cobalt, nickel and neodymium, or in the presence of alkyllithium compounds. The nature of the solvent used in each case depends greatly on the type of catalyst employed. Benzene or toluene and aliphatic or cycloaliphatic hydrocarbons are preferably employed.
The polymerization of unsaturated organic compounds, in particular conjugated dienes, in the presence of catalysts based on rare earth metals has been known for a long time (see e.g. DE-A 28 33 721, U.S. Pat. No. 4,429,089, EP-A 76 535, EP-A 92 270, EP-A 92 271, EP-A 207 558, WO-A 93/05083, U.S. Pat. No. 5,627,119, EP-A 667 357, U.S. Pat. No. 3,478,901, EP-A 637 589). Thus, for example, EP-A 11 184 and EP-A 7027 disclose a catalyst system which is based on rare earth metals, in particular based on neodymium compounds, and is particularly suitable for the polymerization of conjugated dienes, in particular butadiene. In the polymerization of, for example, butadiene, these catalysts give a polybutadiene in very good yields and with a high selectivity, which is distinguished, in particular, by a high content of cis-1,4 units.
It is, furthermore, known to employ anionic initiators, such as butyllithium, for the polymerization of butadiene in hexane. Anionic catalysts are also suitable for a block copolymerization of butadiene with further non-polar monomers, such as styrene and isoprene, or polar monomers, such as ethylene oxide, propylene oxide and acrylates [H. L. Hsieh, R. P. Quirk, Marcel Dekker Inc., New Yorkxe2x80x94Basel, 1996; R. K. Sadhir, R. M. Luck, Expanding Monomers, CRC Press Boca Raton, 1992]. In this case, the butadiene is first polymerized in an inert solvent and, after addition of a further-monomer to the live system, a second block is then formed from the further monomer. The preparation of three-block copolymers is also possible with these anionic initiators.
The disadvantage is that it is not possible, with anionic initiators, to prepare a copolymer with a high cis content, in which the cis-1,4 content of the butadiene block is above 50%, under conditions which are relevant in use.
It is known that compounds for tire mixtures, in particular for treads, are made of several rubbers and fillers in order to achieve an optimum in their properties, such as e.g. the rolling resistance and the abrasion and wet skidding resistance. Polydienes with a high cis content, such as the two synthetic rubbers, polybutadiene and polyisoprene or natural rubber, are preferably employed in these rubbers.
To reduce the rolling resistance, some of the filler carbon black in the xe2x80x9cgreen tiresxe2x80x9d is replaced by silica. One of the main problems of using silica as a filler in rubber is the great difference in polarity between the two components. This results in a poor miscibility. Binding of polar silica to the non-polar rubber matrix has hereto been achieved only by means of coupling reagents, such as e.g. Si-69(copyright) (Degussa AG).
However, for use of the block copolymer as an agent which imparts compatibility of, for example, high cis-BR and silica in vulcanizate mixtures, a low cis-1,4 content in the polydiene part of the block copolymer has an adverse effect on the compatibility of the block copolymer with the high cis-BR rubber matrix and therefore, an adverse effect on the product properties.
In the case of catalysts based on the rare earths, those catalyst systems which allow homopolymerization of polar monomers are also described. Examples of these are the samarium catalyst [(C5Me5)SmH]2 for the polymerization of acrylates [H. Yasuda et al., Macromolecules, 1993, 22,7134; J. Am. Chem. Soc., 1992, 114, 4908; E. Ihara et al., Macromolecules, 1995, 28, 7886] and lactones [M. Yamashita et al., Macromolecules, 1996, 29, 1798] and for the block copolymerization of ethylene with methacrylate or xcex5-caprolactone [H. Yasuda et al., Macromolecules, 1992,25, 5115], the samarium catalyst [(C5Me5)25 mMe] for the formation of tri-block copolymers from variously substituted acrylates [E. Ihara et al., Macromolecules, 1995, 28, 7886] and for the polymerization of cyclic carbonates [H. Yasuda, Prog. Polym. Sci., 2000, 25, 573], the ytterbium catalyst Yb[C(SiMe3)3]2 for the polymerization of methacrylate [H. Yasuda et al., Prog. Polym. Sci., 1993,18,1097; E. Ihara et al., J. Organomet. Chem., 1999, 574, 40] and neodymium catalysts based on Nd(acac)3(H2O)3/AIR3 and Nd(naphthenate)3/AIR3 for the polymerization of lactones [Z. Shen et al., J: Polym. Sci., Polym. Chem. Ed., 1994, 32, 597] and based on Nd(ethyl acetoacetate)2(OPr) for the block copolymerization of cyclic carbonate with lactones [H. Yasuda, Prog. Poly. Sci., 2000, 25, 573].
The preparation of block copolymers with a high cis-1,4-polydiene part and therefore, a low glass transition temperature has not so far been possible.
The object of the present invention was to provide a process for the block copolymerization of conjugated diolefins and polar monomers with which copolymers in which the polymer composition can be varied with respect to the content of conjugated dienes and of polar monomers, at an unchanged high cis-1,4 content in the polydiene content of xe2x89xa760%, are obtained.